The invention relates to a system and method for thermal processing of a workpiece, and more particularly relates to a system and method for regulating a processing temperature of the workpiece, and reducing an amount of process gas required to process the workpiece as well as the corresponding gas switching and purge time.
Devices for thermal processing have been widely known and utilized to perform a variety of thermal processing methods, including annealing, diffusion, oxidation, and chemical vapor deposition. A common workpiece fabricated utilizing such thermal processing devices is a semiconductor wafer. One of ordinary skill in the art understands these thermal processing devices, especially with regard to the impact of process variables on the quality and uniformity of resulting products.
Single wafer rapid thermal processing (RTP) is a known method for physically and chemically treating semiconductor wafers at high temperatures to achieve desired electronic properties for semiconductor devices. The RTP process typically uses two techniques for heating the wafers. In a first technique, a steady-state hot-wall furnace heats a wafer and the wafer temperature is controlled by the mechanical transport of the wafer along a temperature, or thermal radiation, gradient. In a second technique, a wafer is heated by incandescent, or arc, lamps around a cold wall chamber, and the wafer temperature is controlled by varying the optical output of each lamp.
In a second technique, lamp-based RTP systems can heat and cool a wafer at relatively fast ramp-up and ramp-down rates due to the relatively low thermal mass associated with lamps, which offers a low thermal budget (namely the integration of temperature over time) for wafer processing. The temperature control of a wafer inside a lamp-based RTP depends on the fast switching (on and off) of multiple lamps in response to the temperature readings at one or several locations on the wafer. This process necessitates the use of complex optical and electronic feedback and control systems to achieve the required temperature accuracy and uniformity. A wafer being processed is not in thermal equilibrium with its colder surroundings. This, in combination with the random nature of lamp output control, makes the temperature uniformity and reproducibility of a lamp-based RTP difficult. However, since no lateral transport of a wafer inside an RTP chamber is required except for rotation along the wafer normal, a lamp-based system inherently possesses a small chamber volume (about or below 10 liters) relative to a hot-wall-based RTP furnace (about 100 liters). Thus, fast gas switching can be realized by a lamp-based RTP system during a rapid thermal processing cycle, such that a wafer can be consecutively exposed to different ambient gases in synchronization with the wafer temperature. However, known hot-wall RTP systems do not have the advantage of fast gas switching.
A commercially available hot-wall RTP furnace is described in the U.S. Pat. No. 4,857,689 awarded to High Temperature Engineering Corporation, and has been improved by the addition of an in situ emissivity calibration and closed-loop temperature control system as described in U.S. Pat. No. 6,183,127 awarded to Eaton Corporation (SEO).
To further describe hot-wall RTP systems, an upper section of the hot-wall RTP furnace is constantly heated, while a lower section is actively cooled to maintain a steady-state temperature profile. An ambient gas is continuously introduced into the upper section of the furnace and exhausts from the lower section of the furnace. Consequently, a monotonic temperature and thermal radiation gradient exists along the axis of the RTP furnace. The temperature profile is also axially symmetric, with a radial component optimized to ensure the uniform heating of a wafer. Varying the position of the wafer along the temperature gradient controls the heating of the wafer. Since a thermal steady-state is maintained throughout the entire furnace, and between the furnace and the gas ambient, wafer heating is dominated by the thermal equilibration between the wafer and its furnace environment. Consequently, a hot-wall RTP furnace can yield superior results over the lamp-based RTP systems in terms of temperature uniformity, process reproducibility, and cost, while still possessing comparable performance with regard to thermal budget and process throughput. The hot-wall RTP furnace systems have successfully been used in production for implant anneal and activation, silicidation, dry- and wet-silicon oxide growth, diffusion, and metal anneal.
In comparison with the lamp-based RTP systems, however, the hot-wall RTP systems maintain larger furnace internal volumes. This is because a wafer must travel a span of up to 100 cm to make use of the furnace temperature gradient for temperature variation and control. Hence, the furnace must be sufficiently large to accommodate the large sweep volume of the wafer. For example, the sweep volumes for the 200 mm and 300 mm wafers are approximately 31 and 71 liters, respectively, for a 100 cm sweep, leading to a typical furnace volume of about 100 liters. If the fast switching of ambient gases is required for the processes involving the consecutive applications of multiple ambient gases in a RTP cycle, such fast switching can be difficult. In addition, a large chamber volume tends to increase process gas consumption, wafer contamination levels due to the out-diffusing impurities from hot furnace materials, and safety risks because of the quantities of toxic, corrosive, flammable or explosive process gases at high temperatures.
Since the inception of the RTP processing, some development has focused on techniques to increase the temperature ramp-up and ramp-down rates of a wafer to minimize the thermal budget. Additional development has focused on the accurate temperature measurement and control of wafers. The thermal budget of a rapid thermal annealing (RTA) step directly determines the source/drain junction depth and sheet resistance of CMOS devices through defect annealing, re-crystallization, dopant activation, and diffusion in the implanted layers. In addition to the thermal budget control, fast gas switching capabilities are becoming increasingly important in the RTP processes as the vigorous device scaling necessitates the replacement of a silicon oxide (SiO2) dielectric layer with a layered gate dielectric stacks containing silicon oxide, silicon oxynitride (SiOxNy), and silicon nitride (Si3N4), and with high-K dielectric materials in the future. In a two-step RTO process for SiO2 growth (see J. Nulman, J. P. Krusius and P. Renteln, Mat. Res. Soc., Symp. Proc., 52, 341(1985)), for example, a wafer is heated in an oxygen ambient to a preset temperature, and further to a higher temperature, for silicon oxide growth. An RTA is then performed after switching the ambient from oxygen to nitrogen. The RTA step improves the electrical properties of the Sixe2x80x94SiO2 interface.
As another example, the formation of an ultra-thin nitride gate stack by in situ RTP multiprocessing (see S. C. Song, B. Y. Kim, H. F. Luan and D. L. Kwong, M. Gardner, J. Fulford, D. Wristers, J. Gelpey and S. Marcus, Advances in rapid thermal processing, ECS Proceedings of the symposium, V99-100, p45(1999)) requires four consecutive steps in different ambient gases and at different temperatures, namely (1) interface passivation in nitric oxide (NO) gas, (2) silicon nitride (Si3N4) rapid thermal chemical vapor deposition (RTCVD) using silane (SiH4) and ammonia (NH3) at a low pressure, (3) nitridation in ammonia, and (4) anneal in nitrous oxide (N2O). Therefore, the prolonged purge time between two consecutive RTP steps, which is necessary for an RTP chamber with a large internal volume, will reduce the RTP process throughput.
Vertical-type thermal processing furnaces typically support a processing tube within the furnace in a vertical position. The thermal processing furnace also typically employs a workpiece boat assembly, which mounts to appropriate translation mechanisms for moving the workpiece boat into and out of the processing tube or heating chamber. A separate workpiece handling assembly transfers the workpiece from a storage medium to the workpiece boat assembly. One or more workpieces can be accommodated by the workpiece boat assembly. The workpiece boat assembly then selectively raises and positions the workpiece(s) within the heating chamber to at least partially regulate the temperature to which the workpiece(s) is exposed.
There exists in the art a need for a thermal processing apparatus for use with multiple and/or hazardous process gases in a sequential manner to process a workpiece while improving processing throughput. The present invention and example embodiments thereof provide solutions to address this need. Solutions include reducing the effective gas ambient volume surrounding a wafer during processing to shorten the gas purge time for fast gas switching, while still maintaining the RTP process performance of existing RTP systems.
A thermal processing apparatus for processing a workpiece includes a heating chamber in accordance with one aspect of the present invention. A small-volume workpiece enclosure is disposed about the workpiece. A translation mechanism, in the form of a positioning assembly, supports the small-volume workpiece enclosure for moving the small-volume workpiece enclosure and the workpiece within the heating chamber. The heating chamber can have a thermal radiation intensity gradient or a temperature gradient for thermally processing the workpiece. The heating chamber can have one or more heating elements disposed about the heating chamber. The heating chamber can be in the form of a bell jar.
In accordance with another aspect of the present invention, a gas supply can be coupled to the small-volume workpiece enclosure for introducing one or more gases into an interior of the small-volume workpiece enclosure.
In accordance with further aspects of the present invention, a gas diffuser can be disposed within the small-volume workpiece enclosure to at least partially regulate the temperature of the workpiece. A surface of the gas diffuser can have a reflective surface that reflects thermal radiation unabsorbed and emmitted by the workpiece back to the workpiece to at least partially regulate the temperature of the workpiece. The reflective surface can also compensate for the thermal radiation loss due to absorption and reflection by workpiece enclosure materials. The reflective surface can be of uniform, or non-uniform, reflectivity. The term xe2x80x9cgas diffuserxe2x80x9d as utilized herein is intended to describe a component of the thermal processing apparatus that can serve as a diffuser of gases flowing in or around the diffuser, and/or serve as a reflector suitable for reflecting gas and thermal radiation or emissions, depending on the particular arrangement of the gas diffuser within the thermal processing apparatus. The gas diffuser can further include a window formed within the gas diffuser.
A remote sensor, such as a pyrometer, according to a further aspect of the present invention, disposed relative to the small-volume workpiece, can determine the temperature of the workpiece utilizing the window formed within the gas buffer or diffuser. Alternatively, a contact temperature sensor, such as a thermocouple, disposed relative to the small-volume workpiece, can determine the temperature of the workpiece.
According to still another aspect of the present invention, at least one support structure couples to a first section of the small-volume workpiece enclosure, such that when the small-volume workpiece enclosure is lowered to a base portion of the thermal processing apparatus, at least one support structure supports the first section of the small-volume workpiece enclosure. A second section of the small-volume workpiece enclosure is optimally separable from the first section, providing access to an interior portion of the small-volume workpiece enclosure. Alternatively, in a horizontal-type furnace arrangement, a structure couples to a first section of the small-volume workpiece enclosure to aid in separating a first section from a second section, providing access to an inner portion of the small-volume workpiece enclosure.
According to still further aspects of the present invention, the small-volume workpiece enclosure has provided therein at least one aperture formed in a wall, such that gas supplied to the small-volume workpiece enclosure can ultimately escape through the aperture. The heating chamber of the thermal processing apparatus can further include a vent disposed for exhausting gas from the heating chamber.
According to still another aspect of the present invention, an interior wall separates the small-volume workpiece enclosure into a first or outer sub-compartment and a second or inner sub-compartment. The interior wall, according to one aspect, has at least one aperture.
According to still further aspects of the present invention, a gas exhaust is provided in communication with the small-volume workpiece enclosure for exhausting gas in the small-volume workpiece enclosure to a location external to the thermal processing apparatus.
According to one practice, the ratio of the volume of the heating chamber to the volume of the small-volume workpiece enclosure is greater than approximately 2.
The present invention further provides a method of thermally processing a workpiece. The method includes the steps of placing a workpiece to be heated into a small-volume workpiece enclosure, which can be disposed inside a heating chamber. The heating chamber and the small-volume workpiece enclosure are heated, and the small-volume workpiece enclosure is positioned within the heating chamber of the thermal processing apparatus. The workpiece is thermally processed when resident within the heating chamber.
The method according to further aspects of the present invention can include the step of positioning the small-volume workpiece enclosure at one or more positions in the thermal processing apparatus as required to control heating of the workpiece.
According to another aspect of the present invention, the small-volume workpiece enclosure is supplied with a gas (including a sequence of gases) to interact with the workpiece and at least partially regulate heating of the workpiece. The gas can be preheated prior to being supplied to the small-volume workpiece enclosure. Upon entering the small-volume workpiece enclosure, the gas can be partially regulated with a gas diffuser.
According to yet another aspect of the present invention, the method of thermally processing a workpiece housed within the small-volume workpiece includes the step of exhausting gas from the small-volume workpiece enclosure into the thermal processing apparatus. The method can further include the step of venting the thermal processing apparatus to exhaust any gases released from the small-volume workpiece enclosure, or any other process gases. Alternatively, the method can include the step of exhausting the gas through an exhaust line from the small-volume workpiece enclosure. The exhaust line can lead directly out of the thermal processing apparatus.
According to another aspect of the present invention, the method of thermally processing the small-volume workpiece can include the step of exhausting gas to the heating chamber from the small volume workpiece enclosure to dilute, scavenge, or purge the gas from the small-volume workpiece enclosure.
According to another aspect of the present invention, the method of thermally processing the small-volume workpiece can include the steps of using plasma and photon energizing devices to energize process gases prior to entering the small volume workpiece enclosure.
In accordance with still another aspect of the present invention, a thermal processing apparatus for processing a workpiece is provided. The thermal processing apparatus includes a heating chamber having at least one of a thermal radiation intensity gradient and a temperature gradient. A small-volume workpiece enclosure is disposed about the workpiece, and a positioning assembly that supports the small-volume workpiece enclosure and moves the small-volume workpiece enclosure and the workpiece to desired locations is disposed within the heating chamber to subject the workpiece to different heating levels. The thermal processing apparatus is capable of performing a dry or wet rapid thermal oxidation technique, rapid thermal nitridation technique, rapid thermal anneal technique for implant diffusion and activation of metal silicides, rapid thermal BPSG reflow technique, selective oxidation technique of Si in the presence of a metal, rapid thermal chemical vapor deposition technique, low pressure chemical vapor deposition technique, metal-organic chemical vapor deposition technique, remote-plasma chemical vapor deposition technique, and multi-layer dielectric gate stack formation technique.